The present disclosure relates to gas sensors, and particularly to the electrolyte of a sensor.
The automotive industry has used exhaust gas sensors in vehicles for many years to sense the composition of exhaust gases, namely, oxygen. For example, a sensor is used to determine the exhaust gas content for alteration and optimization of the air to fuel ratio for combustion.
One type of sensor uses an ionically conductive solid electrolyte between porous electrodes. For oxygen, solid electrolyte sensors are used to measure oxygen activity differences between an unknown gas sample and a known gas sample. In the use of a sensor for automotive exhaust, the unknown gas is exhaust and the known gas, (i.e., reference gas), is usually atmospheric air because the oxygen content in air is relatively constant and readily accessible. This type of sensor is based on an electrochemical galvanic cell operating in a potentiometric mode to detect the relative amounts of oxygen present in an automobile engine""s exhaust. When opposite surfaces of this galvanic cell are exposed to different oxygen partial pressures, an electromotive force (xe2x80x9cemfxe2x80x9d) is developed between the electrodes according to the Nernst equation.
With the Nernst principle, chemical energy is converted into electromotive force. A gas sensor based upon this principle typically consists of an ionically conductive solid electrolyte material, a porous electrode with a porous protective overcoat exposed to exhaust gases (xe2x80x9cexhaust gas electrodexe2x80x9d), and a porous electrode exposed to a known gas"" partial pressure (xe2x80x9creference electrodexe2x80x9d). Sensors typically used in automotive applications use a yttria stabilized zirconia based electrochemical galvanic cell with porous platinum electrodes, operating in potentiometric mode, to detect the relative amounts of a particular gas, such as oxygen for example, that is present in an automobile engine""s exhaust. Also, a typical sensor has a ceramic heater attached to help maintain the sensor""s ionic conductivity. When opposite surfaces of the galvanic cell are exposed to different oxygen partial pressures, an electromotive force is developed between the electrodes on the opposite surfaces of the zirconia wall, according to the Nernst equation:   E  =            (                                                  -              RT                                                                          4              ⁢              F                                          )        ⁢          ln      ⁡              (                                                            P                                  O                  2                                ref                                                                                        P                                  O                  2                                                                    )            
where
E=electromotive force
R=universal gas constant
F=Faraday constant
T=absolute temperature of the gas
PO2ref=oxygen partial pressure of the reference gas
PO2=oxygen partial pressure of the exhaust gas
Due to the large difference in oxygen partial pressure between fuel rich and fuel lean exhaust conditions, the electromotive force (emf) changes sharply at the stoichiometric point, giving rise to the characteristic switching behavior of these sensors. Consequently, these potentiometric oxygen sensors indicate qualitatively whether the engine is operating fuel-rich or fuel-lean, conditions without quantifying the actual air-to-fuel ratio of the exhaust mixture.
For example, an oxygen sensor, with a solid oxide electrolyte such as zirconia, measures the oxygen activity difference between an unknown gas and a known reference gas. Usually, the known reference gas is the atmosphere air while the unknown gas contains the oxygen with its equilibrium level to be determined. Typically, the sensor has a built in reference gas channel which connects the reference electrode to the ambient air. To avoid contamination of the reference air by the unknown gas, the sensor requires expensive sensor package that usually has complex features in order to provide sufficient gas sealing between the reference air and the unknown gas. Alternatively, in-situ electrochemical oxygen pumping can be used. In this method, the air reference electrode chamber is replaced by a sealed reference electrode with oxygen electrochemically pumped in from the exhaust gas. This method eliminates the exhaust gas contamination problem but creates its own drawbacks. That is, an expensive electronic circuit is required to do the electrochemical oxygen pumping.
Manufacturing techniques used to create gas sensors continue to evolve with the goal of providing a more durable sensor that will be resistant to cracking as a result of temperature cycling, while decreasing the cost. Accordingly, there remains a need in the art for a low cost, temperature resistant sensor.
The drawbacks and disadvantages of the prior art are overcome by the gas sensor electrolyte and method for making the same. The electrolyte comprises up to about 80 wt % monoclinic zirconia, up to about 30 wt % stabilizer, and up to about 40 wt % dopant-zirconia.
The method of making an electrolyte, comprises blending monoclinic zirconia powder with a co-precipitated stabilized zirconia and stabilizer to form a mixture; and forming an electrolyte from the mixture.